What if we have mass resignations from cabinet, plus a leadership challenge (for which 20% of the party would have to support a challenge)?
Resignations:
John Prescott, Deputy Prime Minister and First Secretary of State (OTL had doubts over the legality of the invasion)
Gordon Brown - Chancellor of the Exchequer and Second Lord of the Treasury (OTL believed the Prime Minister was too 'gung ho' over Iraq, according to Tam Dalyell)
Robin Cook, Lord President of the Council and Leader of the House of Commons (OTL resigned)
Jack Straw, Secretary of State for Foreign and Commonwealth Affairs (OTL considered resigning over Iraq)
Patricia Hewitt, Secretary of State for Trade and Industry and Minister for Women and Equality (OTL raised objections to the Prime Minister about his policy toward Iraq, according to Robin Cook)
David Blunkett, Secretary of State for the Home Department (OTL raised objections to the Prime Minister about his policy toward Iraq, according to Robin Cook)
Clare Short, Secretary of State for International Development (OTL resigned)
Brown would be the obvious candidate for a challenge, though it's a broad spectrum, politically, and the leadership vote won't come in time to stop the vote on the war. Encouraged by these resignations, more Labour MP might vote against war on Iraq. Then there's the spectre of the majority of the party voting for war on Iraq, but the war going ahead thanks to the support of the Tories. We only need 216 Labour MP to vote against the war, and the motion is defeated...I think it's possible but unlikely.